In order, therefore, to decide what is the nature of a precipitate produced by the hydrolysis of a normal salt, it is only necessary to ascertain whether and how the composition of the precipitate alters with alteration in the composition of the solution. If the composition of the solution is represented by abscissæ, and the composition of the precipitate by ordinates, the form of the curves obtained would enable us to answer our question; for vertical lines would indicate the presence of two solid phases (1st case), horizontal lines the presence of only one solid phase (2nd case), and slanting lines the presence of mixed crystals (3rd case). This method of representation cannot, however, be carried out in most cases. It is, however, {298} generally possible to find one pair or several pairs of components, the _relative amounts_ of which in the solution or in the precipitate undergo change when, and only when, the composition of the solution or of the precipitate changes. Thus, in the case of bismuth, nitrate, and water, we can represent the ratio of Bi_{2}O_{3} : N_{2}O_{5} in the precipitate as ordinates, and N_{2}O_{5} : H_{2}O in the solution as abscissæ. A horizontal line then indicates a single solid phase, and a vertical line two solid phases. An example of this is given in Fig. 118.[372] [Illustration: FIG. 118.] Bi_{2}O_{3}--N_{2}O_{5}--H_{2}O.--Although various systems have been studied in which there is formation of basic salts,[373] we shall content ourselves here with the description of some of the conditions for the formation of basic salts of bismuth nitrate, and for their equilibrium in contact with solutions.[374] Three normal salts of bismuth oxide and nitric acid are known, viz. Bi_{2}O_{3},3N_{2}O_{5},10H_{2}O(S_{10}); Bi_{2}O_{3},3N_{2}O_{5},4H_{2}O(S_{4}); and Bi_{2}O_{3},3N_{2}O_{5},3H_{2}O(S_{3}). Besides these normal salts, there are the following basic salts:-- {299} Bi_{2}O_{3},N_{2}O_{5},2H_{2}O (represented by B_{1-1-2}) Bi_{2}O_{3},N_{2}O_{5},H_{2}O ( " " B_{1-1-1}) 6Bi_{2}O_{3},5N_{2}O_{5},9H_{2}O ( " " B_{6-5-9}) 2Bi_{2}O_{3},N_{2}O_{5},H_{2}O ( " " B_{2-1-1}) Probably some others also exist. The problem now is to find the conditions under which these different normal and basic salts can be in equilibrium with solutions of varying concentration of the three components. Having determined the equilibrium conditions for the different salts, it is then possible to construct a model similar to that for MgCl_{2}--KCl--H_{2}O or for FeCl_{3}--HCl--H_{2}O, from which it will be possible to determine the limits of stability of the different salts, and to predict what will occur when we bring the salts in contact with solutions of nitric acid of different concentrations and at different temperatures.